Modern adhesives cure at a very slow rate, sometimes in the order of days. It is known to speed the curing of these adhesives as well as improving increased bond strength by heating the adhesives. Various means exist for heating adhesives to aid in their curing.
Several common means of heating adhesives include conduction, convection and radiant heating of the adhesive. Conduction heating is achieved by placing a wire in the adhesive and passing a current through the wire. The current heats the wire which in turn heats the adhesive. Conduction heating has the disadvantage of requiring a conductive wire to be placed in the adhesive bond. The wire is difficult to be properly located within the adhesive and weakens the bond.
Radiant or convection heating is achieved by placing the part to be bonded with adhesive in a radiant or convection oven. The adhesive is heated indirectly by heat passing through the parts. Convection or radiant heating requires that both the parts and the adhesive be heated in an oven. Some parts bonded with adhesives such as thermoplastics, may weaken or deform when heated to the temperatures necessary to cure the adhesive. Further, radiant and convection heating generally require upwards of twenty minutes to cure the adhesive.
Another method of curing adhesives by concentrating heat within the adhesive is taught and disclosed in U.S. Pat. No. 4,749,833, issued Jun. 7, 1988 to Novorsky et al. Novorsky et al. teaches induction heating of an adhesive. Spherical particles of steel are placed within the adhesive and moved in rolling contact with one another to establish an accurate space between two members to be bonded in an adhesive joint. The adhesive joint is placed between or adjacent to an induction coil and a current is passed through the coil. The current passing through the coil induces the spherical particles to heat thereby heating the surrounding adhesive. As in the method of conduction heating, induction heating also requires the addition of foreign metal particles within the adhesive thereby decreasing the strength of the bond.
It is known to heat polar materials including adhesives in a high frequency electric field by a process called dielectric heating. U.S. Pat. No. 3,291,671, issued Dec. 13, 1966 to Hecht, teaches a fusing of plastic films by dielectric heating. A water containing paper board separated by one or more polyethylene films is placed between two electrodes. A radio frequency (RF) generator is attached to the electrodes and passes an electric field through the polyethylene film and water containing paper board. The polyethylene films fuse to one another and to the paper board. This illustrates dielectric heating but not the dielectric heating of an adhesive to promote curing.
Dielectric heating to cure adhesives is shown and disclosed in EPO Patents 0,339,494 and 0,339,493, both filed Apr. 20, 1989 and U.S. patent application Ser. No. 07/187,358 filed May 28, 1988 now U.S. Pat. No. 4,941,936. These patents teach the bonding of a fiber-reinforced plastic (FRP) exterior member to a U-shaped FRP reinforcement member. A bead of two-part epoxy resin adhesive is placed between the exterior and reinforcement members. The bonded assembly is then moved to a chamber containing a dielectric heater. The first electrode having roughly the same contour as the outer skin is placed against the outer skin member and a second electrode having a concentrator is placed over the reinforcement member. A high-frequency electrostatic field of between 300 and 8,000 volts is applied through the electrodes. The high frequencies range between 25 and 40 MHz. This apparatus teaches curing times of approximately 30 to 40 seconds. The apparatus uses a plurality of electronically isolated concentrators rather than a single elongated electrode. The patents teach that an elongated electrode used to bond parts exceeding 47 inches at 34 MHz experiences uneven heating. This is believed to result from peaks of energy radiating from the elongated electrode as the length approaches 1/15th of the wavelength of alternating voltage. The EPO patents require a separate electrode for both surfaces of the members being bonded.
The EPO patents teach away from the use of long electrodes (i.e., electrodes longer than 1/15th the RF wavelength) by teaching the need for a plurality of isolated concentrators. Additionally, the EPO patents do not teach the use of a conductive frame as an electrode.
It is a primary object of this invention to utilize RF dielectric heating for bonding a member to a conducting frame. The conducting frame is used as one electrode of the RF circuit. Nonuniform heating of elongated plates is reduced by pulsing the RF electric field. Adhesive cure can be monitored and controlled by the plate current between the electrodes thereby minimizing the time and energy necessary for dielectric heating.